SLVAF38 March 2021 TPS61096A

2 Operating Principle

The Figure 2-1 shows a boost converter circuit with a coupled transformer. The turns ratio of the transformer is 1:N, the inductance value of the primary side is L1 and the inductance value of the secondary side is L2. C1 and C4 are input and output capacitor, Q1 is the N-MOS and D2 is rectifier diode integrated in the TPS61096A, D1 is rectifier diode.

Figure 2-1 The Boost Converter Circuit with Coupled Transformer

According to the inductor current value at the end of each switching cycle, the circuit could operate at CCM(Continuous conduction mode), BCM(Boundary conduction mode) and DCM(Discontinuous conduction mode). Figure 2-2 is the ideal waveform of the converter when the boost converter operates in CCM.

Figure 2-2 CCM Waveform

During one switching cycle T_S, the primary current and secondary current have two working status. When the MOSFET Q1 is turned on and the diode D1 is off, the inductor current rises linearly from I_{L1_0} to I_{L1_1} as shown in Equation 1.

Equation 1.

The MOSFET Q1 is turned off and the diode D1 is on. The current through L1 suddenly decreases, and the energy in the primary inductor is transferred to the secondary. The initial current of the secondary is shown in Equation 2, where the N is the turn ratio of the coupled inductor.

Equation 2.

For the next switching cycle, the primary inductor continues to store energy, and then transferred the energy to the secondary. If the input voltage and the output current keep stable, the I_{L1_4} is equal to I_{L1_0}. At the steady state, each winding of coupled inductor meets the voltage-second balance, as shown in Equation 3.

Equation 3.

Equation 4.

From Equation 3, Equation 4, the duty cycle in CCM mode is shown in Equation 5.

Equation 5.

Figure 2-3 shows the device operates in BCM. When the device operates in the BCM mode, the duty cycle D is the same as Equation 5.

Figure 2-3 BCM Waveform

Figure 2-4 shows the device operates in DCM. When the device operates in the DCM mode.

Within D₁*T_S, the N-MOS is on and the inductor current increases linearly from zero.
Within D₂*T_S, the N-MOS is off and the inductor current decreases to zero.
In the rest of a switching cycle, both the N-MOS and D1 is off. The V_SW is equal to V_IN ideally.

Figure 2-4 DCM Waveform